Quantitative recovery of krypton from gas mixtures mainly comprising carbon dioxide

ABSTRACT

A continuous process for the quantitative recovery of krypton from a feed gas mainly comprising carbon dioxide and also containing minor percentages of oxygen and nitrogen, as well as trace quantities of krypton. The process includes three principal separations: absorption of krypton from the feed gas to provide a krypton-decontaminated waste gas; fractionation of the gases coabsorbed with the krypton; and stripping of the krypton from the absorbent to provide (a) krypton-free carbon dioxide for use as the process absorbent and (b) krypton-enriched liquid product. The carbon dioxide absorbent is derived from the feed gas itself.

Ferguson et al.

[73] Assignee: The United States of America as represented by the UnitedStates [57] ABSTRACT Atomic Energy Commission, Washington, DC. Acontinuous process for the quantitative recovery of [22] Filed Jul 251972 krypton from a feed gas mainly comprising carbon diy oxide and alsocontaining minor percentages of oxygen [21 Appl. No.: 275,001 andnitrogen, as well as trace quantities of krypton. The

1 process includes three principal separations: absorption of kryptonfrom the feed gas to provide a kryptondecontaminated waste gas;fractionation of the gases [58] Fie'ld 612/17 23 co-absorbed with thekrypton; and stripping of the /24 krypton from the absorbent to'provide(a) kryptomfree carbon dioxide for use as the process absorbent and (b)krypton-enriched liquid product. The carbon dioxide [56] gtsxsgrabsorbent is derived from the feed gas itself. 7 I 2,551,399 5/1951 fSilverberg 62/20 10 Claims, 2 Drawing Figures I -sa WASTE GAS G; 0.295Uml gggoxeK CONDENSER 0-29OX "i 7 lint] -4oc 995%02 FEED CONDENSER99.96%N IN FEED '-40c 9 v L 0.496 1.56 FEED GAS 15 672? 1 AMOUNTI 1 MOLEBURNER OFF-GAS Max lpervunit 0.897 MOLE C02 E time] 0.0750 MOLE 0 g 530.0280 8kg 5 3w c e 3 K r a x Xe 21-22%. Y 1 I 5 0.412x g-fi 3 L 5 m :3I m l Ell lfi To /l g 3 1 71 K L AND 3 9 a 6.44X

CD 0 cooL TO 5 -4oc 4 EL is 8222. O 266 X m m V 6m 1 GL L L56 9 5.7 LE-2 252%. i 2 522x 7.25 E & L

QUANTITATIVE RECOVERY OF KRYPTON FROM GAS MIXTURES MAINLY COMPRISINGCARBON DIOXIDE Inventors: Don E. Ferguson; Paul A. Haas, both ofKnoxville; Rex E. Leuze, Lenoir City, all of Tenn.

2,962,868 l2/l960 Dennis 62/20 2,502,282 3/1950 Schlitt 62/22 PrimaryExaminer-Norman Yudkoff Assistant Examiner-Arthur F. PurcellAttorney-Roland A. Anderson 3,742,720 [451 July 3, 197a PATENIEDJUL31913 37421720 isnanurz" 5 10- p.- O LI.

2 CALCULATED o (IDEALL z 5 q. LL! (0 CALCULATED (NONIDEAL) 2 -6O 4O -20E O 20 4O TEMPERATURE (C) SOLUBILITY 0F KR m LIIQUIID 00 GAS MIXTURESMAINLY COMPRISING CARBON DIOXIDE This invention was made in the courseof, or under, a contract with the United States Atomic EnergyCommission. I

This invention relates generally to processes for the separation of anoble gas from a mixture of gases, the noble gas being more volatilethan some of the constituent s of the mixture and less volatile thanothers.More specifically, this process relates to recovering radioactivekrypton in highly concentrated form from a gas mixture which mainlycomprises carbon dioxide 'but which also contains smaller percentages ofoxygen and nitrogen as well as trace quantities of krypton-85 and xenon.

A previously proposed method for the re-processing of irradiated nuclearfuel calls for burning all or part of the 'carbon blocks which containthe fuel. The off-gas from the burning operation consists mainly ofcarbon dioxide (CO and contains minor percentages of oxygen (O andnitrogen (N as well as trace quantities of radioactive krypton (Kr),xenon (Xe), and iodine In some instances, the off-gas also may contain'asmall percentage of carbon monoxide (CO).

The above-mentioned method for fuel re-processing calls for delaying theburning of the irradiated fuel blocks until sufficient time has elapsedfor the decay of much of the radioactive iodine and essentially all ofthe radioactive xenon. This simplifies processing of the offgas fromthe. burning operation, since the iodine then can be removed bycomparatively simple techniques and since removal of the xenon from theoff-gas no longer is necessary. After an iodine-removal step, the burneroff-gas would be decontaminated with respect to the' l(r (half-life,10.76 years) and then disposed of by release to the atmosphere.Preferably, the Kr would be recovered continuously in a highlyconcentrated (i.e., low-volume) form suitable for economical, long-termstorage.

Quantitative recovery of the Kr from the burner offgas is a difficultand unusual problem because the principal constituent, CO is in variousrespects physically similar to Kr. Furthermore, the off-gas includessome constituents which, unlike C0,, are more volatile than Kr. Forexample, following storage and iodine removal the off gas may compriseapproximately 90 mole C0 7 mole 0 3 mole N 1 mole CO; and tracequantities of Xe (10-100 ppm) and Kr (3 to ppm). As mentioned, the Kr ismore volatile than the CO, but less volatile than the N 0 and CO.

Various continuous methods have been considered for the quantitativerecovery of the radioactive Kr from the burner off-gas. As used herein,quantitative recovery" refers to recovery of at least 95 percent of theKr in the feed gas to the process. Because of the physical similarity ofKr and CO separation by means of membrane permeation, adsorption, orphysical absorption is not attractive. Moreover, separation by means ofthermal diffusion, hot-metal trapping or chemical reaction does notappear practical. Various processes are liquefaction would not provide asuitably concentrated product. Removal of the Kr from the off-gas canbeaccomplished by extraction into a fluorocarbon solvent, but thisextraction is not attractive where CO is present in large amounts, sincethe CO would concentrate with the krypton. Low-temperature distillationmight be used to separate purified CO from the other components of theoff-gas. Since CO may be present, such distillation would need to bepreceded by an operation for the removal of CO or 0, in order to ensurethat the subsequent removal of CO by distillation would not result inCO-O concentrations within explosive limits. A potential probleminherent in kryptonremoval processes conducted at liquid-oxygentemperatures is the, formation of solid CO in the process lines orcolumns. Another problem, imposed by the presence of appreciable O isthat the liquefaction of Kr along with appreciable amounts of 0 mayproduce hazardous amounts of O, by radiolysis. Still another approach tothe problem of recovering the Kr from burner off-gas would be to employhot K CO solution for the selective removal of the CO but this leavesthe Kr unseparated from the more volatile N 0 and CO.

OF THE INVENTION Accordingly, it is an object of this invention toprovide a novel process for the quantitative recovery of krypton fromkrypton-containing gas mixturesprincipally comprising carbon dioxide.

It is another object to provide a process for selectively recoveringkrypton from a krypton-containing gas mixture principally comprisingcarbon dioxide and also containing minor amounts of oxygen and nitrogen.

It is another object to provide a process of the kind described whereinthe krypton is recovered in lowvolume form. Q

It is another object to provide a krypton-recovering process which doesnot require operation at liquidoxygen temperatures.

Other objects will be made apparent hereinafter.

This invention can be summarized as follows: A method for recoveringkrypton in highly concentrated form from a gas mixture constitutedmainly of carbon dioxide and including a relatively small percentage ofoxygen as well as a trace amount of krypton comprising (a) compressingand cooling said gas mixture to convert the same to a two-phase mixtureincluding a gaseous fraction and'a liquid fraction, both fractionscontaining carbon dioxide, oxygen, and kypton; (b) separating saidgaseous fraction and liquid fraction; (c) passing said gaseous fractionupwardly through an absorption zone in countercurrent contact withkryptondecontaminated liquid carbon dioxide absorbent ultimately derivedfrom said two-phase mixture to preferentially transfer krypton into saidabsorbent; (d) separately withdrawing the resultingkryptondecontaminated gaseous effluent from said zone; (e) passing saidliquid fraction and the krypton-enriched liquid absorbent from saidabsorption zone downwardly through a fractionating zone incountercurrent contact with carbon dioxide vapor ultimately derived fromsaid two-phase mixture to transfer oxygen into said vapor; (f) recyclingat least a part of the resulting oxygen-enriched gaseous effluentupwardly through said absorption zone; (g) feeding the resultingoxygendepleted, Krypton-containing liquid absorbent from saidfractionating zone downwardly through a stripping zone in countercurrentcontact with carbon dioxide vapor ultimately derived from said two-phasemixture to transfer krypton into said vapor; (h) recycling at least apart of the resulting krypton-decontaminated liquid absorbent downwardlythrough said absorption zone; (i) rectifying the resultingkrypton-enriched vapor effluent from said stripping zone to provide avapor stream further enriched in krypton; and (j) recovering as productat least a part of said stream further enriched in krypton.

BRIEF DESCRIPTION OF THE DRAWINGS gas evolved by the burning ofirradiated graphite fuel blocks.

DESCRIPTION OF TI-I PREFERRED EMBODIMENT This invention will beillustrated in terms of a continuous process for the recovery of Kr froma feed gas having the following composition per mole: CO 0.897 mole;0.0750 mole; N 0.0280 mole. The feed gas also contains trace quantitiesof Kr and Xe; these trace quantities are expressed herein as K mole andX mole, respectively. A feed gas of this composition is representativeof burner off-gas (see above) which has been stored for a selectedperiod and treated to remove certain other constituents. The process tobe described is based on the preferential solubility of Kr in liquid COand is illustrated in terms of calculated ideal separation factors basedon extrapolation of the Kr vapor pressure above the critical point.

In the process to be described, the liquid CO absorbent is derived fromthe feed gas itself. The entire process system is conducted underpressure and tem perature conditions ensuring that solidification of COis avoided. The process involves three principal separations havingdifferent liquid-gas flow rate limitations, and thus it is conductedconveniently in at least three liquid-gas contactors. By means of thethree principal separations, the Kr is removed not only from the muchlarger quantities of the less-volatile CO but also from the relativelylarge amounts of more-volatile O and N The particular form of theprocess to be described yields a low-volume product stream whichcontains (in terms of feed gas concentrations) about 99% of the Kr, lessthan I% of the CO and less than 1% of the 0,. More than 99% of the input0 and N is vented as a waste gas decontaminated with respect to Kr.Liquid CO, in excess of that required to maintain a stable inventory isdischarged from the system as a stream rich in Xe but decontaminatedwith respect to Kr.

The accompanying flow diagram (FIG. 2) and the supplementary tableappearing hereinafter illustrate the invention as employed for thetreatment of one mole of the above-described feed gas per unit time. Asshown, the feed gas first is compressed and cooled in any suitableapparatus 1 to liquefy a major portion (73.8 percent) thereof. Theresulting two-phase mixture GL is fed to the bottom portion 4 of anabsorption column 5, the bottom stage of which separates the mixture GLinto a gaseous stream G, and a liquid stream Lp. As shown, a recycledgas stream G from a fractionating column 3 also is fed into the bottomof the absorption column 5. To indicate the division of the twophasestream GL into streams G and L,.-, a phase separator 2 is shown;actually, this separator is the bottom stage of absorption column 5. InFIG. 2 the flow rate of the stream G is expressed as 0.262; 0.6851(; and0.266X. These figures indicate, respectively, the total moles flow perunit time of those constituents other than the trace elements Kr and Xe;the Kr flow in terms of K moles per unit time; and the Xe flow in termsof X moles per unit time.

The absorption column 5, which is designed with nine theoretical stages,can be of any suitable type,

such as a conventional tower filled with standard packing. The gasupflow in this column comprises stream G F and the above-mentionedrecycle stream G,,. The liquid effluent from this column comprisesstreams L and L these two streams, together with a stream of condensateL,,, are fed to an inlet 6 in the top portion of the fractionator 3. Thegas introduced to the bottom of the absorption column 5 is contactcountercurrently with Kr-deeontaminated, liquid CO absorbent introducedat an upper inlet 15. The contact is effected in an absorption zonemaintained at a temperature of 40C and a pressure of 15 atmospheres.This operation is designed to scrub approximately 99% of the Kr from thegas. The resulting gaseous effluent G which contains more than 99% ofthe N, and O in the original feed gas, is decontaminated sufficientlyfor discharge to atmosphere. The downflowing liquid CO absorbent removesXe even more effectively than it does Kr; because of recycle of thestream G however, the waste gas G, contains about 29% of the originalXe.

The fractionating column 3 effects quantitative removal of 0 and N, fromthe Kr-containing liquid introduced at the upper inlet 6. Thefractionating column is of standard design and may be a conventionalpacked tower having seven theoretical stages. The column is providedwith an overhead condenser 7 for receiving a gaseous stream g it is alsoprovided with a reboiler 8 for receiving a liquid stream L, from thebottom of the column. The feed to the upper inlet 6 of the columnconsists of the absorption column liquid effluent L,,, theaforementioned liquefied fraction L,.-, and condensate L from thecondenser 7. As this feed flows down through the column, nearly all ofthe 0 and N, dissolved therein is transferred into counterflowing COvapor from the reboiler 8, introduced to the column through a bottominlet 9. In the fractionator, a part of the Kr in the downflowing liquidis transferred to the vapor. As shown, the gaseous effluent g from thecolumn is fed to the condenser 7. The major part of the Kr in thecondenser feed is recycled to the absorption column 5 in the gaseouseffluent G and the remainder is refluxed to the fractionator in thecondensate stream L As shown, the fractionating zone is maintained at 15atmospheres and about 28C. A stream L of liquid is withdrawncontinuously from the fractionator reboiler 8 and fed to a side inlet 10of a tripping column 11. As shown in the table, stream L is mainly COthe stream also contains appreciable amounts of dissolved Xe and Kr butonly small fractions of the 0 or N Expressed in terms of the originalfeed gas concentrations, the function of the stripping column i l is toconcentrate more than 99% of the Kr into about 1% of the C0 The strippercan be of any suitable design and may be a packed rectification columnhaving nine stripping stages below the feed inlet '10 and two enrichingstages above. The stripper is provided with an overhead condenser 12 forreceiving a vapor stream V, and with a reboiler 13 for receiving aliquid stream L from the bottom of the column. A stream V of vapor fromthe reboiler is introduced to the column through a lower inlet 14. Thestripper is maintained at atmospheres and 40C.

In the stripping stages of column 11, the Krcontaining, liquid feed L iscontacted countercurrently with vapor from the lower inlet 14. As aresult, virtually all of the Kr is transferred into the vapor. Theresulting liquid CO effluent L contains only about 0.3% of the Kr in thefeed gas, but, because of Xe recycle, about 8 times as much Xe as thefeed gas.

A liquid stream L is withdrawn continuously from the bottom of thereboiler 13. As shown in the table, this stream is highly pure COcontaining about 0.09% of the Kr in the feed gas and about 5.5 times theoriginal amount of Xe. A major part L of this stream is recycled to theupper inlet of the absorption column -5 for re-use as the processabsorbent. Thus, the process absorbent is CO derived originally from theabovementioned two-phase mixture GL obtained by partial liquefaction ofthe feed gas. To maintain a stable inventory of CO in the overallsystem, the remainder of stream L is withdrawn as a waste stream Lwhich, as shown in the table, contains less than 0.01% of the originalKr.

The stages above the feed stage in column 11 concentrate the Kr in theCO vapor. As shown, the Kr content of the vapor stream V, from the topof the column is 172 times that of the feed gas to the system. The vaporV is liquefied in the condenser 12 and is refluxed as stream L to thetop of the stripper via a top inlet 23. A selected fraction of thereflux stream is withdrawn as the product stream L,,. In terms of thecomposition of the feed gas to the system, the product L contains morethan 99% of the original Kr, less than 1% of the CO about 4%of the Xe,and less than 1% of the O and N The composition of stream L,, is shownin the table.

Referring again to the absorption operation, the flow rate of the liquidCO is made large enough to absorb virtually all (e.g., 99%) of the Krbut not so large as to absorb more than a selected percentage of the Oand N,. In the absorption operation illustrated in FIG. 2, whichprovides a Kr decontamination factor of about 100, the L/V flow ratio isabout 10 moles of CO per mole of vapor. From the standpoint ofquantitative removalof Kr, the minimum effective flow of absorbent isabout 9 moles of CO per mole of vapor, since the Kr/CO, vapor pressureratio is about 9. If desired, additional Kr absorption can be achievedby increasing the column operating pressure or the flow of liquid C0,.Either of these changes will, however, increase the amount of O and Ncarried to the fractionator in the absorbent stream L Quantitativereturn of the resulting larger amounts of 0, and N in the recycle stream(I, from the fractionator will increase the amount of Kr refluxing inthe system. Increasing the number of absorption stages may be apreferred way of increasing Kr absorption since this increases the Krdecontamination factor without increasing the amount of N and Oabsorbed. For each tenfold increase in the decontamination factor, aboutfive additional theoretical stages are required.

The absorption operation can be conducted over a wide range oftemperatures and temperatures selected to maintain the CO absorbent inthe liquid phase. Throughout the entire process system, the pressuremust be above 5.1 atmospheres and the temperature above 56.6C (thefreezing point of CO at that pressure). The separation factors between 0and Kr increase with decreasing temperature; thus, it is preferable tooperate at a temperature (e.g., 40C) ap-- proaching the minimum. Theseparation factors also increase with increasing pressure.

Referring to the fractionator 3, this contactor is designed to remove O,and more volatile gases dissolved in the liquid CO absorbent receivedfrom the absorption column. in the fractionator, the vapor-to-liquidmole ratio is made large enough to remove the desired percentage (e.g.,more than 99%) of these gases but not so large as to transfer more thana selected percentage of the Kr to the vapor. Kr transferred to thevapor places an additional burden on the absorption column, since partof the vapor is refluxed to that column as stream G in the system shown,the Kr reflux to the absorption column from the fractionator is about1.4 times the rate that Kr is introduced to the system in the feed gas.If preferred, the reflux of Kr can be reduced by decreasing thevapor-to-liquid mole ratio in the fractionator. This will, however,increase the number of stages required to completely remove the O and Nfrom the absorbent. The seven-stage fractionator shown decreases O inthe liquid C0 to about 0.7% of its concentration in the feed gas. Ifdesired, additional stages can be added to further decrease the amountof dissolved 0 This will slightly increase the amount of Kr refluxed tothe absorber. The fractionator is more effective in removing N and 0,.Less Xe than Kr is refluxed from the fractionator to the absorber.

In the stripper i ll, the two stages above the feed stage greatlyconcentrate the Kr in the reflux loop at the top of the stripper.Virtually all of the O and N coming from the fractionator also isconcentrated in this loop. if a smaller reflux of Kr is preferred, theamountof vapor and liquid in these upper stages can be decreased.Alternatively, the vapor from the fractionator feed stage can berectified in a small, separate column. A part of the vapor furtherenriched in krypton by rectification is recovered as product, as bycondensation and withdrawal from the system.

An important feature of this process is that it effects quantitativerecovery of the Kr in a form essentially free from more-volatile gasessuch as 0 and N This is achieved by employing both an absorptionoperation and a fractionation operation, with recycle, prior to thestripping operation. Merely absorbing before stripping poses the problemthat the absorber can be operated to favor either good Kr recovery or alow 0, content in the liquid leaving the absorber, but not both. Thatis, conditions effecting complete absorption of the Kr would also resultin absorption of at least 20% of the 0, present, whereas conditionseffecting absorption of less than about 10% of the would not accomplishabsorption of more than about 50% of the Kr.

it will be understood that the process conditions illustrated above arenot necessarily the optimum and that passing said gaseous fractionupwardly through an absorption zone in countercurrent contact withkrypton-decontamined liquid carbon dioxide absorbent ultimately derivedfrom said two-phase it is within the scope of this invention to vary thepromixture to preferentially transfer krypton into said cess parametersas requiredto obtain a desired Kr deabsorbent; contamination factor, aselected Kr concentration in separately withdrawing the resultingkryptonthe product stream, a desired flow rate for the productdecontaminated gaseous effluent from said zone; stream, or th like. F xp in a system analogous passing said liquid fraction and thekrypton-enriched to that shown in FIG. 2, a Kr decontamination factorliquid absorbent from said absorption zone down- Of about 1,000 can beObtained with absorption, wardly through a fractionating zone incountercurtionation, and stripping columns having ten theoretical rentcontact with carbon dioxide vapor ultimately stages each and operatingat about -C an 20 atm derived from said two-phase mixture to transferoxspheres. Assuming a feed gas comprising at least 90 ygen i t id vapor;mole CO and including up to 10 o 5 P 2, 5 recycling at least a part ofthe resulting oxygenas well as 1 to 100 ppm Kr, quantitative recovery ofthe ri hed gaseous effluent upwardly through said Kr in low-volume formcan be obtained within the folb ti zone; lowing ranges of operatingconditions: feeding the resulting oxygen-depleted, kryptonb so 30 Ccontaining liquid absorbent from said fractionating A sorption zone:temperature to pressure 6 m 70 mm 20 zone downwardly through a strippingzone in coun- L/V ratio 3 to 20 tercurrent contact with carbon dloxldevapor ulti- Fracmnal'ng tempemure C mately derived from said two-phasemixture to pressure 6 to 70 atm. L/v ratio 5 30 transfer krypton intosaid vapor; Stripping lonel temperature I recycling at least a part ofthe resulting kryptonjl; s a d decontaminated liquid absorbentdownwardly through said absorption zone; I g. to other well-known typesof rectifying the resulting krypton-enriched vapor efflugas contactorscan be used if desired. For example, m f om i stripping Zone to id avapor bubble-plate columns or packed columns with true stream furth i hd i k d countercurrent flow can be utilized. As further examrecoveringas product t least a pal-t of id stream ples, spray towers or jetcontactors might be used. furth i h d i krypton in the form of the pr cej desfl'lbed, the Xe 2. The method of claim 1 wherein said gas mixtureis Permitted to distribute throughout the Process Systemcompressed andcooled under conditions liquefying a If desired, however, virtually allof the Xe can be conmajor ortio thereof centrated in the product streamL,,, along with the Kr 3, The method of claim 1 wherein each f said yutilizing a much larger pp and p y g sorption zone, fractionating zone,and stripping zone is q i /v p r fl w ra i of less h two n the rippmaintained at a pressure in the range of about 5.2 to 70 This mode ofoperation will greatly increase the Kr reatmospheres and a temperaturein the range of from flux at the top of the stripper. about 57 to 30C.

H TABLE Liquid phase Vapor phase Stream composition (moles) Streamcomposition (moles) Stream Total C02 02 N2 Kr' Xe Stream Total 002 02 N2Kr Xe Feed gas..... 1.00 0.807 0.0750 0. 0280 K 0.00028 0.00251 0.3151;0.734X GF 0. 252 0.1704 0. 0557 0.0255 0.685K 0.205x

10- 10- 0.0008K 4.82X 0.0505 0.0155 0.811K 5.07X 0.537 0. 340 0.1445 0.0435 0.516K 0.531); 0.0505 0.0155 2.10K 5.22X 0.537 0.340 0.1445 0.04351.3151: 0.54814 0. 53s 2.111; 0.678X 0. 00313 0.00050 0184K 0.320X 0.2750.1786 0 0788 0 0180 1.421K 0.412X

0.0819 0.01580 2.60K 027x 0.00104 0.000027 1.661K 023x g1 0.405 0. 4030.00105 0. 000000 1.035K 0.720X 0. 000515 0.000010 0.991K 554x gn 0.4050.404 0.00104 0.000027 0.671K 0.696X 0.000515 0.00001 0.0005K 0.037x

10- 10+ 0.0033K 7.83X v1 1. 1. 550 l0- 10- 0.00050: 2 00x 10- 10-0.0000K 5.50): v], 1.50 1.500 10- 10- 0.0023K 003x 10- 10- 0.0001K0.673X

Liquid and Vapor Phases on the same line are in equilibrium. Basis: 1mole of contaminated burner gas per unit of time.

We claim:

1. A method for recovering krypton in highly concentrated form from agas mixture constituted mainly of carbon dioxide and including arelatively small percentage of oxygen as well as a trace amount ofkrypton comprising:

compressing and cooling said gas mixture to convert the same to atwo-phase mixture including a gaseous fraction and a liquid fraction,both fractions containing carbon dioxide, oxygen, and krypton;separating said gaseous fraction and liquid fraction;

4. The method of claim 1 wherein the liquid-to-gasphase flow rate ratioin said absorption zone is in the range of from about 3 to 20.

5. The method of claim 1 wherein the liquid-to-gas phase flow rate ratioin said fractionating zone is in the range of from about 5 to 30.

6. The method of claim 1 wherein the liquid-to-gasphase flow rate ratioin said stripping zone is in the range from about 1.5 to 10.

7. The method of claim 1 wherein a part of said oxygen-enriched gaseouseffluent from said fractionating zone is cooled to form a condensatewhich is recycled 8. The method of claim 1 wherein said carbon dioxidevapor contacted in said fractionating zone is gener- 9. The method ofclaim 1 wherein part of said kr.yp-

ton-decontaminated liquid from said stripping'zone is separatelydischarged as waste.

10. The method of claim 1 wherein said vapor stream ated by distillationof said oxygen-depleted absorbent 5 further enriched in krypton iscondensed and a part of from said fractionating zone.

the resulting condensate is withdrawn as said product.

2. The method of claim 1 wherein said gas mixture is compressed andcooled under conditions liquefying a major portion thereof.
 3. Themethod oF claim 1 wherein each of said absorption zone, fractionatingzone, and stripping zone is maintained at a pressure in the range ofabout 5.2 to 70 atmospheres and a temperature in the range of from about-57* to 30*C.
 4. The method of claim 1 wherein the liquid-to-gas-phaseflow rate ratio in said absorption zone is in the range of from about 3to
 20. 5. The method of claim 1 wherein the liquid-to-gas phase flowrate ratio in said fractionating zone is in the range of from about 5 to30.
 6. The method of claim 1 wherein the liquid-to-gas-phase flow rateratio in said stripping zone is in the range from about 1.5 to
 10. 7.The method of claim 1 wherein a part of said oxygen-enriched gaseouseffluent from said fractionating zone is cooled to form a condensatewhich is recycled downwardly through said fractionating zone.
 8. Themethod of claim 1 wherein said carbon dioxide vapor contacted in saidfractionating zone is generated by distillation of said oxygen-depletedabsorbent from said fractionating zone.
 9. The method of claim 1 whereinpart of said krypton-decontaminated liquid from said stripping zone isseparately discharged as waste.
 10. The method of claim 1 wherein saidvapor stream further enriched in krypton is condensed and a part of theresulting condensate is withdrawn as said product.